When laser light is launched in an optical fiber it is often required that the power of the light entering the fiber be estimated by way of being monitored. Generally a known portion of the light is extracted and detected, and this detected portion provides an indication of the power of the total laser light propagating within the optical fiber.
Numerous methods and systems have been envisaged for tapping a portion of the light received from a laser propagating within an optical fiber. Some of these methods are complex and costly to implement.
For example, U.S. Pat. No. 6,778,585 in the name of Malone, assigned to Optical Communication Products, Inc., incorporated herein by reference, relates primarily to VCSELS. Within optical communication systems utilizing VCSELs, or in fiber laser systems or diode laser systems it is important to control the optical data signal being transmitted. Because VCSELs which are commonly used in today's optoelectronics industry, emit a unidirectional light beam normal to the surface in which they are formed, it is especially difficult to monitor the output optical power of the VCSEL which provides the data signal without attenuating or otherwise compromising the integrity of the light emitted. In order to control the output optical power of a VCSEL and to maintain the optical power at a desired level, the output optical power must first be monitored. Based on the monitored output optical power, adjustments can be made to the current signals supplied to the VCSEL to control the output optical power of the VCSEL which provides the data signal.
As described in U.S. Pat. No. 6,778,585, refracted ray coupling techniques, also known as “Refracted Near Field” techniques, are used in the field of optoelectronics to characterize optical fibers, the contents of which are herein incorporated by reference.
In many laser systems, when an optical source, such as a laser, is coupled to an optical fiber, preferably most of the light propagates as guided rays in the core of the optical fiber. The portion of light, entering the core under an angle, larger than the critical angle, is not totally reflected, but is partially refracted into the cladding. The amount of this light depends of mismatch between numerical aperture (NA) of the focusing system and NA of the optical fiber. The light, refracted into cladding, is once more refracted on the boundary between cladding and outer cover and, partially reflected, penetrates into the cover layer and, finally exits into the free space outside of the fiber. The portion of the light, remaining in the cladding, creates high order cladding modes, but, since the reflection angle for these modes is smaller than critical angle, they are strongly leaking both into core and outside. Therefore, their propagation length is very short. U.S. Pat. No. 6,778,585 suggests using this light, refracted from the cladding and, finally, coming out of fiber, to monitor power inside the fiber. A complex and costly light collection system, in the form of a refracted ray coupler, is disclosed as a means of monitoring this light exiting the core and the cladding. This design suggested in U.S. Pat. No. 6,778,585, incorporated herein by reference, essentially uses the properties of refracted rays. These rays are well directed in the limited spatial angle; and, they emerge out of fiber only at very short distance from the fiber input.
There is one important aspect, which is not discussed in U.S. Pat. No. 6,778,585, which we believe is important for function of the device, described therein. The scheme is characteristic for multimode fibers with large core diameter. Having a large core is most practical for this type of light coupling and resultant light collection from light that has escaped the core through the cladding.
In contrast, this invention provides a solution that is particularly useful with single mode optical fibers and wherein a small portion of light launched directly into the cladding from an end of the optical fiber and propagating in the cladding as low order modes is monitored and wherein the requirement for a refracted ray coupler is obviated.
Furthermore this invention does not require bending the optical fiber to tap light therefrom. However, a section of optical fiber with a bend radius of less than 20 mm may be used for monitoring light within the optical fiber.
It is therefore an object of this invention to provide a method for monitoring cladding launched light by directly detecting portions of the cladding launched light by using a detector disposed adjacent to and along a side of the fiber, utilizing the spatially homogeneous distribution of light scattered from the cladding.
It is a further object of this invention to provide a single mode optical fiber carrying light to be monitored by a photodiode positioned so that light exiting the fiber is detected after having propagated only though free space between the fiber and a photodiode.